1. Field of the Invention
The present invention relates to a negative electrode material containing a magnesium compound and a battery using the same.
2. Description of the Related Art
With progress of electronic engineering in recent years, many small portable electric devices, such as video cameras combined with video tape recorder, cellular phones, and laptop computers, become widely used, and the miniaturization and the weight saving thereof are attempted. Then, as a portable power supply used for these devices, development of small and lightweight secondary batteries with a high energy density is required strongly. As a secondary battery which theoretically has a high energy density and can generate a high voltage, batteries using a light metal such as a lithium metal, a sodium metal, or an aluminum metal as a negative active material are known, and the practical application thereof is expected. Especially, a lithium secondary battery using a lithium metal as a negative active material has excellent handling and can achieve high output power and a high energy density. Therefore research and development has actively been done.
However, if the above light metal such as the lithium metal is directly used as a negative electrode material, the light metal will easily precipitate into a dendrite on the negative electrode in a charging process, and the current density become very high at the tip of the dendrite. Because of this, the cycle life decreases due to the degradation of electrolytes and the like, and the dendrites grow excessively which results in electric short-circuit in the battery. Then, in order to prevent such precipitation of the metal in the dendrite, instead of directly using the lithium metal for the negative electrode, a graphite material utilized for the intercalation reaction of the lithium ions between graphite layers, or a carbonaceous material applied to the occluding/releasing action of the lithium ions inside pores is used.
However, with the graphite material utilized for the intercalation reaction, the negative electrode capacity has an upper limit as specified in the composition C6Li of the first stage graphite intercalation compound. Moreover, although the carbonaceous material applied to the occluding/releasing action inside the pores has not such a theoretical upper limit, it is industrially difficult to control such a minute pore structure, and the increase in number of the pores can cause the reduction of the specific gravity of the carbonaceous material, and the carbonaceous material cannot serve as an effective mean of improvement in the negative electrode capacity per unit volume. For such a reason, it is difficult for the conventional carbonaceous materials to adapt to electric devices for longer time utilization and higher energy density of the power supply thereof in the future, and it is desired to develop the negative electrode material which is excellent in the occluding/releasing capability of lithium ions.
As other negative electrode materials, the magnesium silicide (Mg2Si), tin-magnesium intermetallic compound (Mg2Sn), germanium-magnesium intermetallic compound (Mg2Ge), and lead-magnesium intermetallic compound (Mg2Pb), which have reverse fluorite structure, are known, for example. However, these compounds have the problem that the cycle characteristic as the negative electrode is low (H. Sakaguchi, H. Honda and T. Esaka, J.Power Sources, 81–82, 224 (1999)).